1. Field of the Invention
This invention relates to a method of cutting needle threads and a looper thread in interlocking with the stoppage of the sewing operation in a multi-needle covering chainstich sewing machine with a cylinder bed, and also to a thread trimming device which is useful for performing the method.
2. Description of the Related Art
FIG. 1 is a view illustrating sewing steps performed in a multi-needle covering chainstich sewing machine. The multi-needle covering chainstich sewing machine has a plurality of needles 1 which are arranged in parallel in the front side of a throat plate disposed on a bed (not shown), and a looper 2 in the rear side of the throat plate. In the sewing machine, the sewing operation is performed on a material fabric A which is fed on the upper face of the throat plate along a direction orthogonal to the arrangement direction of the needles 1.
The needles 1 and needle threads la passing through the tip of the respective needles 1 as shown in the figure reciprocate vertically with the operation of feeding the material fabric A, in a predetermined moving stroke which penetrates the upper and lower faces of the throat plate. The looper 2 has a working rod 2b through which a looper thread 2a is passed from the rear end to the front end of the rod as shown in the figure. The looper 2 performs a rotational motion over a predetermined angle range and about a rotational axis which elongates along the feeding direction of the fabric A. This rotational motion is caused in interlocking with the feed of the fabric A. In this rotational motion, the working rod 2b moves or swings along the given circumference of a circle, the center of which is the rotational axis, to reciprocate along the parallel-arrangement line of the needles 1.
FIG. 1(a) shows the state in which the needles 1 are at the lowest position (bottom dead point), and FIG. 1(c) the state in which the needles 1 are at the highest position (top dead point). FIG. 1(b) shows the transition state from the bottom dead point to the top dead point, and FIG. 1(d) shows that from the top dead point to the bottom dead point. As seen from these figures, the working rod 2b performs one advance motion while the needles 1 move upward, and one retracting motion while the needles 1 move downward. The fabric A is intermittently fed by a predetermined length every downward movement of the needles 1, i.e., every transition from FIG. 1(c) to FIG. 1(d).
In the part of the process from FIG. 1(a) to FIG. 1(c) via FIG. 1(b), loops of needle threads 1a are formed directly under the needles 1 with the upward motion thereof, respectively. The working rod 2b which advances during this process is inserted into these loops, together with the looper thread 2a passed through the rod. As a result, in the state of FIG. 1(c) wherein the needles 1 reach the top dead point, the respective needle threads 1a are temporarily arrested by the working rod 2b of the looper 2.
In contrast, in the process from FIG. 1(c) to FIG. 1(a) via FIG. 1(d), the needle threads 1a arrested by the working rod 2b of the looper 2, and the looper thread 2a passed through the working rod 2b are caused to be respectively pulled out in a relative manner by the length corresponding to the one feeding length, by the operation of feeding the material fabric A which is performed during this process. In this case, the looper thread 2a is obliquely pulled out from the rear end of portions of intertwisting with the needle threads 1a and toward the front end of the working rod 2b, so as to form a triangle as shown in FIG. 1(d). During this process, the needles 1 which move downward after passing through the material fabric; A are sequentially passed through the vertex area of the triangle formed by the looper thread 2a, and then rectangular areas which are defined by the respective needle threads 1a and the looper thread 2a, so that, as shown in FIG. 1(a), the looper thread 2a is pulled out from the working rod 2b of the looper 2 which retracts at this time. The needles 1 hold the looper thread 2a in the state in which the needles 1 are passed through the loops formed by the needle threads 1a.
By repeating the above-mentioned operations, on the front side of the fabric A, parallel seams of the needle threads 1a are formed, and, on the back side of the fabric A, seams are formed as shown in FIG. 1 in which the loops of the needle threads 1a passing to the back side of the fabric A are correspondingly related with the looper thread 2a passing through the loops and intertwisted with them. FIG. 1 shows the stitch configuration in which top cover seaming is formed also on the front side of the fabric A. The top cover seaming on the front side is formed by the operation of a top cover thread looper 3 which moves as illustrated with the vertical motion of the needles 1.
In order to improve the sewing efficiency, such a multi-needle covering chainstich sewing machine for forming stitches as described above is generally provided with a thread trimming device which moves with the completion of a series of sewing operations to collectively cut the needle threads 1a and the looper thread 2a. The thread trimming device includes a fixed knife which has an edge facing the passing route of the needle threads 1a and is fixed to the lower face of the throat plate, and a movable knife which reciprocates in the arrangement direction of the needle threads 1a while sliding along the fixed knife.
The movable knife has engaging claws at its front end and middle portion which cooperate with the edge of the fixed knife to perform the shearing operation when retracting. The movable knife conducts one reciprocating motion at the time that the needles 1 are stopped at the top dead point after the sewing operations are ended. The needle threads 1a and looped thread 2a at the top dead point are shown in FIG. 1(c). Under this state, the movable knife enters from the side of the working rod 2b into the loops of the needle threads 1a through which the working rod 2b of the looper 2 has been passed. The engaging claw which is disposed at the front end and elongates to the vicinity of the front end of the working rod 2b engages with the looper thread 2a, and the engaging claw disposed at the middle portion engages with the innermost needle thread 1a. Thereafter, the movable knife retracts along the advance motion route to return to the initial position wherein the engaging claw disposed at the front end overlaps with the edge of the fixed knife.
During this retracting motion, the plural needle threads 1a are collectively pulled out by the engaging claw disposed at the middle portion of the movable knife, and the looper thread 2a is pulled out by the engaging claw disposed at the front end. These pulled out threads are cut by the shearing action of the edge of the fixed knife. Since this retracting motion is terminated when the movable knife returns to the initial position, the looper thread 2a which has been pulled out by the engaging claw disposed at the front end remains to be pressingly held between the fixed and movable knives after the cutting operation, and therefore there is no fear that the looper thread 2a slips off the working rod 2b of the looper 2. This eliminates the necessity of setting again the looper thread 2a to the looper 2 for the next sewing operation.
When the needles 1 are at the top dead point, a thread tension releasing mechanism (not shown) works with the motion of the needles 1 to relax the tension applied to the needle threads 1a and looper thread 2a, so that the afore-said pulling out operation accompanied with the next feed of the material fabric A can be done. This allows the afore-said pulling out operation for cutting these threads to be performed without causing any problem.
Commercially manufactured multi-needle covering chain-stich sewing machines are provided with a bed which has a shape selected from a wide variety of shapes according to the kind of the products to be sewn. For the purpose of sewing a raglan sleeve, for example, multi-needle covering chainstich sewing machines with a cylinder bed of a small diameter sufficient for being inserted into the sleeve are widely used.
In a case that such a multi-needle covering chainstich sewing machine with a cylinder bed is to be further provided with a thread trimming device such as described above, however, it is often that there is not sufficient space for additionally installing a movable knife and a mechanism for driving it, in the bed which is positioned under the throat plate and in which the looper 2 moves. To cope with this, therefore, it is required to improve the configuration.
A thread trimming device for a sewing machine of this kind, i.e., for a multi-needle covering chainstich sewing machine with a cylinder bed is disclosed in Japanese Utility Model Application Laid-Open No. 63-139879 (1988). This thread trimming device employs a movable knife which has a hook-like curved shape and is provided with one engaging claw at its front end. The base end portion of the movable knife is rotatably supported at the lower face of the throat plate and in the downstream of the needle drop points, so that the movable knife is swingable about the supporting axis in a plane parallel to the throat plate. An operating end of an actuator such as an air cylinder which reciprocates in the direction of the axis of the cylinder bed is engaged with the vicinity of the supporting axis, so that the movable knife is swung by the operation of the actuator.
In the same manner as the usual thread trimming device described above, the improved thread trimming device starts its operation under the state that the needles 1 are stopped at the top dead point. Initially, the movable knife advances along its curved line with the advance operation of the actuator, and enters into loops of the needle threads 1a which are arranged under the needle drop points as shown in FIG. 1(c). At this time, the engaging claw disposed at the front end of the movable knife engages with the innermost needle thread 1a. Then, owing to the retracting motion of the actuator which follows the advance motion, the other needle threads 1a and the looper thread 2a are collectively arrested by the engaging claw disposed at the front end of the movable knife, to be pulled out. These needle threads 1a and looper thread 2a are cut by the combination of the movable knife and the fixed knife which slidingly contacts with the movable knife at the end of the retraction route, and then pressingly held between the knives.
In the thread trimming device, the movable knife is driven by the actuator which may be appropriately disposed on the extension line of the axis of the cylinder bed, and the space to be prepared at the back side of the throat plate is that required for the swing motion of the arcuate movable knife which is performed within a predetermined angle range and along the curved line of the movable knife. Therefore, the thread trimming device can be installed inside the cylinder bed of a small diameter, which leaving plenty of empty space, but has drawbacks described below.
The cut needle threads 1a which are pressingly held between the movable knife and the fixed knife are pulled out from their respective held positions by the removal of the fabric A which has been subjected to the sewing operation. In this case, also the looper thread 2a which is pressingly held together with the needle threads 1 may be pulled out, with the result that the looper thread 2a may slip off the working rod 2b of the looper 2 at the start of the next sewing operation. When this happens, it is impossible to form seams and the sewing operation must be stopped. Therefore, it is necessary to take an adequate action for preventing the looper thread 2a from slipping off the looper 2.
In a sewing machine with a cylinder bed of a small diameter, the motion range of the looper 2 is restricted, and therefore a working rod 2b having a large curvature such as shown in FIG. 1 cannot be used. To comply with this, the front end of the working rod 2b is positioned below loops formed by the needle threads 1a, so that the front end of the movable knife which is passed through these loops as described above is positioned above the looper thread 2a elongating from the front end of the working rod 2b. Accordingly, the engaging claw disposed at the middle portion can engage with the needle threads 1a, but the engaging claw disposed at the front end cannot engage with the looper thread 2a.
Japanese Patent Applications Laid-Open No. 63-214277 (1988) and No. 63-262182 (1988) discloses thread trimming devices wherein a movable knife which has the substantially same curvature as the working rod 2b of the looper 2 and is curved in the direction of thickness is swingingly arranged on an axis that is coaxial with the rotation axis of the looper 2, a fixed knife is disposed in the route of the swing motion, and the movable knife performs one reciprocating motion under the state that the needles 1 are stopped at the top dead point, thereby cutting the needle threads 1a and looper thread 2a.
According to this configuration, the movable knife moves along the working rod 2b of the looper 2, and therefore the looper thread 2a can be surely arrested by the front end of the working rod 2b. However, the above-discussed problem that, when the needle threads 1a are pulled out with the removal of the material fabric A, also the looper thread 2a which is pressingly held together with the needle threads 1a may be pulled out and the next sewing operation is hindered, remains to be solved.